Surface acoustic wave devices are known to the prior art. In one form these devices comprise a pattern of conductors on a piezoelectric substrate. Usually the pattern of conductors comprises a pair of parallel opposed, spaced conductors each of which has a number of so-called interdigital electrodes orthogonal to and connected to the conductor. The interdigital electrodes have a length which is less than the spacing between the conductors and are all parallel to one another. Adjacent ones of the interdigital electrodes are connected to alternate ones of the conductors.
In this configuration an electric field produced by reason of a voltage difference between the conductors will generate a surface acoustic wave in the substrate due to its piezoelectric characteristic. The prior art has utilized these devices as filters in a two port four terminal device which includes a pair of transducers. The first transducer generates a surface acoustic wave by reason of a voltage applied to it via a pair of the terminals. The second transducer converts the surface acoustic wave generated by the first transducer to an electrical signal which is then coupled to the second pair terminals. By adjusting the materials of the transducers and the parameters thereof these devices can be made to have impedance characteristics which are frequency sensitive and thus can perform a filtering function.
Although these devices are useful they do have a number of drawbacks. For one thing, each requires a pair of surface acoustic wave transducing means to be deposited on the substrate. Furthermore, they require a certain amount of piezoelectric material to couple the surface acoustic waves generated by one transducer to the other transducer. Furthermore, there is a desire to sharpen the impedance characteristics of the device so that it could be used, for instance, as a tank circuit to form a basic building block for filter and oscillator applications.
Another frequency selective device which employs acoustic waves is the bulk-wave transducer. In this device, an electric field generated between two spaced electrodes is coupled through a piezoelectric material. In this application the frequency response of the device is very sharp. However, since the electric field is applied across the piezoelectric material the frequency response is directly dependent upon the thickness of the material between the electrodes. As a result accurate dimensions for the piezoelectric material are essential. Furthermore, as the desired frequency range of the device increases the thickness of the device must decrease. With present day technology the frequency range for bulk-wave piezoelectric devices is about 20 megahertz with an upper limit of 100 megahertz.